I used the musubi mold to make thin (1/2" thick) hamburger patties.
Season (I used Montreal Steak Seasoning), then fry. They'll shrink some as they cook, so when you flip them press down on the patties with the spatula to spread them out a little bit. When done, set on paper towels to drain well.

Build your musubi by putting a burger in the mold, then a slice of american cheese torn to fit. Top with rice, mixed with some pickle and/or onion if you want. Compress the whole thing and wrap in plastic wrap.

We don't care for nori, so the cheese serves to lock the burger to the rice. I used American because it's what I had, but any good melty cheese will work. I have some in the freezer as a test, but I don't see any reason why these won't freeze as well as spam musubi.